1. Field of the Invention
The present invention relates to the field of active fluid heat exchanger devices, and more precisely to those which contain a two-phase fluid and which include capillary channels.
2. Description of the Related Art
The term “two-phase” means that the fluid contained in such devices is present in the form of two phases, a liquid and a gas, in order to enable such devices to operate.
The term “capillary” is used below to mean channels of section that is very small relative to their length, and above all which are suitable for producing pumping phenomena by capillarity on liquids.
Two-phase fluid heat exchanger devices are already known that have capillary channels suitable for producing phenomena of capillarity on the liquid phase, and having gas transport channels in which the gas phase of the fluid is confined, the capillary channels communicating with the gas transport channels.
Such devices are used either as closed-loop devices or as open-loop devices.
In closed-loop operation, the device is used as a heat pipe and it functions in self-contained manner. In that application, the device is exposed to a cold zone for condensation, also known as a “cold source”, and to a hot zone for vaporization, also known as a “hot source”. The fluid condenses into its liquid phase in the cold zone and it is vaporized into its gas phase in the hot zone. Capillary forces act on the liquid phase of the fluid to move it from the condensation zone to the vaporization zone. Since gas pressure is higher in the vaporization zone than in the condensation zone, a flow of gas is obtained in the direction opposite to the movement of the liquid phase. The capillary and pressure forces on their own act to drive fluid circulation.
In open-loop operation, the device is used as an evaporator, and a pump and a condenser are integrated in the circuit. For the device to be operational, the fluid must arrive in liquid form in the device and leave it in gas form to be condensed in some other element of the circuit. In the presence of gravity forces, it suffices to orient the device appropriately in order to conserve the liquid in a liquid arrival zone in the device, given that liquid is denser than gas and cannot move away via the gas circuit downstream from the device. However, in the absence of gravity forces, the liquid takes up the form of droplets dispersed in the gas phase. The capillary channels then serve to fix the droplets and to prevent them moving away along the gas circuit downstream from the device.
In such applications, devices of a first type are already in use which are constituted by cylindrical rods of circular section stacked in a hexagonal array perpendicularly to their long direction. When stacked in this way, the rods define cavities between one another. These cavities extend longitudinally parallel to the rods and present a cross-section that is roughly triangular. These cavities contain the two-phase fluid. Those portions of the outside surfaces of the rods that are situated in the vicinity of the apexes of the triangles, i.e. close to the contact zones between pairs of rods, constitute channels suitable for exerting capillary forces on the liquid phase of the fluid. The central zone of each cavity forms a gas transport channel. In order to enable that type of device to operate properly, it is essential for there to be no interruption of the capillary channels along their length. This requires the cylindrical rods to be stacked accurately and rigidly. The rods are therefore received and held in grooves formed in a rigid and rectilinear bar. A device of that type is relatively expensive to make and presents drawbacks in certain applications. For example, its rigidity constitutes one of its drawbacks since that is poorly compatible with allowing pieces to which it is fixed to move when such pieces are subjected to stresses. In addition, when such a device is used as a heat pipe, its performance depends on its capacity to transport heat by means of the fluid. Fluid displacement in the heat pipe is driven by capillary forces exerted on the liquid phase of the fluid as contained in the capillary channels. However, in devices of that type, a large amount of volume is occupied by the cylindrical rods themselves. Consequently, the number of capillary channels for any given volume is relatively small, thus limiting the performance of such devices. This lack of compactness does not provide for good integration with electronic circuits to which the device is fitted.
By way of example, document FR 2 735 565 discloses another type of device. A device of this other type is constituted by aluminum tubes that are internally fluted to form capillary channels that open out to a hollow central region that serves as a gas transport channel. In that case also, the cylindrical shape of the tubes does not favor optimum compactness or performance.
Proposals have also been made, for example in document U.S. Pat. No. 5,697,428, for a device in which a continuous furrow is etched in a first metal plate. The furrow has rectilinear portions that are parallel to one another and that are interconnected by curved portions, the overall shape being zigzag. A second metal plate is placed on the first plate so as to close the furrow and form a tube. In such a structure, the zones where the fluid is in its liquid phase and those where the fluid is in its gas phase follow one another along the path of the fluid in the tube. The inside dimensions of the tube remain the same in all of the zones where the fluid moves. That device therefore cannot optimize the circulation of each phase of the fluid independently. In particular capillary pumping is not achieved and, in that case, the term “capillary” relates essentially to the shape of the tube whose cross-section is very small compared to its length, thereby enabling the gas to remain in the form of bubbles in the liquid and to push it.